HOUSTON -- The only thing that's holding Astros rookie outfielder George Springer back in his rehab from a quad injury that has caused him to miss 12 games is being able to run. Of course, running is a huge part of Springer's game, so the Astros are being cautious.

Springer took batting practice on the field for the fourth day in a row on Friday and has been shagging balls in the outfield. But until he can run at full speed without any issues, Springer is not going to be sent on a Minor League rehab assignment.

"That's my goal," Springer said. "I've been doing stuff slowly every day, and it's kind of built up. You have to start small."

Astros general manager Jeff Luhnow wasn't sure if Springer would travel with the team to Philadelphia next week or stay behind and rehab in Houston, or whether Springer was ready for a rehab assignment.

"He can hit, he can throw, and we're not concerned about that," Luhnow said. "It's a matter of whether or not his quad is going to act up when he runs full speed. It's about getting him comfortable when he runs full speed and stopping and starting, and make sure he's OK with that."

Manager Bo Porter said before Friday's game against the Blue Jays that Springer probably could have played through the leg injury.

"But at the same time, getting the rest we were able to get him and the extra treatment, it's going to allow him to come back and play at full strength the rest of the season," Porter said.